In screw machines such as refrigerant compressors, the refrigerant being compressed tends to move the screw rotors towards the suction side and away from the discharge side. In the case of tri-rotor compressors the sun rotor has a much larger diameter than the other rotors and this equates to a much larger area to be acted on by the discharge pressure. In the case of a tri-rotor, the sun rotor has about 150xc2x0 of compression with each of the coacting rotors and about 30xc2x0 of overlap with each coacting rotor. Suction and discharge pressure are separated at the discharge end face of the sun rotor by a distance corresponding to the extremes of the overlap distance. Accordingly, discharge pressure does not act over the entire discharge end face of the sun rotor and suction pressure can act over part of the discharge end face of the sun rotor with a relatively short distance between discharge and suction pressure. In addition to the thrust loading produced by the discharge pressure acting on the ends of the rotors, the separation of the rotors from the discharge side represents a leak passage. The discharge side bearings and related structure tend to severely limit movement of the rotors away from the discharge and thereby limit leakage. Commonly assigned U.S. Pat. No. 5,975,867 discloses structure associated with the discharge side bearings for limiting axial movement of the screw rotors. The suction side bearings are much less loaded due to the movement restraint applied to the rotors by the discharge side bearings and their related structure. U.S. Pat. No. 5,911,743 discloses balancing the pressure on the ends of the rotors to limit thrust loading of the bearings. This approach requires radial porting with a reduction in port area and efficiency as well as additional parts.
Pressure balancing on the ends of a screw rotor is achieved by locating a fluid pressure chamber at the discharge end of the screw rotor and exposing the chamber to suction pressure. The fluid pressure chamber is sealed from the discharge pressure acting on the outer portions of the discharge end of the screw rotor by a labyrinth seal located between the discharge end of the rotor and the facing housing structure. In addition to providing a fluid seal, the labyrinth reduces leakage between the discharge end of the rotor and the housing. The labyrinth seal and the fluid pressure chamber are both located between the rotor profile root diameter and the shaft diameter. The actual design of the labyrinth and fluid pressure chamber is a compromise of a number of mutually exclusive goals. The actual screw machine dictates some dimensional limits upon which the following goals are superimposed: (1) a desire to have as much labyrinth seal as possible; (2) a desire to have the outer diameter of the labyrinth seal as large as possible; (3) the desire to have the inner diameter of the labyrinth seal as large as possible; (4) the desire to have a greater port area than is available when a thrust disk is employed; and (5) the desire to have a simpler design than that of a thrust disk.
It is an object of this invention to reduce thrust loading on a sun rotor of a multi-rotor screw compressor.
It is an additional object of this invention to provide pressure balancing while employing axial porting.
It is another object of this invention to reduce leakage at the discharge end of a screw rotor. These objects, and others as will become apparent hereinafter. are accomplished by the present invention.
Basically, the area of the discharge end of a screw rotor acted on by the discharge pressure is reduced by providing a region of suction pressure which acts on the discharge end of the rotor and separating the suction and discharge pressures by a labyrinth seal located between the discharge end of the rotor and the facing housing structure.